Maybe you don't believe in hijab because you don't understand what its true purpose is. When applying any Islamic principle in our daily lives we have to first consider this: Do I, or the people around me, have more knowledge than the knowledge of Allah (سبحانه و تعالى)? Would my creator and sustainer ever decree something that wouldn't ultimately benefit me and the society around me? What's sorely lacking here is an understanding of the first principle of Islam, which is evident in its very name. Islam means Submission. Not submission to society and fashion trends, not submission to beauty, but submission to Allah. Everything in Islam was ordained with a purpose, even if, with our limited knowledge, that purpose isn't clear to us. But before you ask for evidence, ask yourself this: "Who am I to question an order from my creator?" Acceptance of Allah's orders should always come first. Shouldn't Allah's superior knowledge of all things be enough for me to accept His ordinance? But Islam doesn't ask us to follow religion blindly, every part of our religion is backed with reasoning.
There are abundant practical purposes for hijab, but many forget the MAIN purpose. To identify ourselves as Muslims. "And tell the believing women to draw their outer garments around them...in order that they may be known (to be Muslims)." (33:59)
What we wear may seem like something frivolous, something that doesn't stand for anything, but in truth it stands for much more than we may realize. What I wear is a way of expressing who I am to the world, without saying anything. I can convey my social status, my interests, and my personality all through my clothing. In a way, it is our identity. A business man always wear a tailored suit to work. A doctor can't walk around the hospital in sweat pants, he has to wear scrubs. Teenagers identify themselves by wearing a picture of their favorite band on their t-shirt. Not wearing hijab is akin to stating that you don't wish to identify yourself as being Muslim.
With hijab I cover my beauty and my looks, because those things are irrelevant. The way I look doesn't say anything about who I am as a person, it doesn't represent my character, my piety, my morals or my intellect. So by covering up the external beauty I highlight what really matters: my internal beauty. And I demand to be judged by only those things, not by society's beauty standards.
I wear hijab because my relationship with God is the most important thing to me, and I want it to be the FIRST thing you know about me when you look at me. It is the only thing I wish to be defined by.
And for those who still say they "Hate hijab," I can only say this:
وَعَسَىٰ أَن تَكْرَهُوا شَيْئًا وَهُوَ خَيْرٌ لَّكُمْ وَعَسَىٰ أَن تُحِبُّوا شَيْئًا وَهُوَ شَرٌّ لَّكُمْ وَاللَّهُ يَعْلَمُ وَأَنتُمْ لَا تَعْلَمُونَ
"But perhaps you hate a thing and it is good for you: and perhaps you love a thing and it is bad for you. And Allah knows, while you know not." (2:216)